1. Field of the Invention
The present invention relates to a digital switching network for switching digital channels that serve in establishing communications for multiservices such as telephony, data transmission, videotelephony, etc.
Switching systems have already been put forward for multiservice digital networks suited to hybrid switching or, in other words, for switching time division multiplex circuits by synchronous digital time division switching networks and for packet switching using asynchronous packet switching networks. These switching systems do not allow:
1. circuits with any given bit-rates to be switched since a synchronous time division multiplex switching network has just a single bit-rate, e.g. 64 kbit/s for a PCM time division multiplex switching system having frames of 125 .mu.s containing octets of 8 bits.
2. A variable proportion of circuits and packets to be switched unless of course each switching network (circuit or packet) is dimensioned for the overall maximum bit-rate processed by the switching system.
These switching systems therefore imply, in practice, a rigid association between service classes and switching techniques (in this case voice switching corresponds to circuit switching and switching of data corresponds to packet switching) which limits the possibilities of future development bound to new economic optima (e.g. packetized voice switching) or the introduction of new services (e.g. low bit-rate data circuit-mode switching).
An asynchronous time division switching system makes it possible, however, to switch a variable proportion of time division circuits and miscellaneous bit-rate packets by generalizing their processing in one and the same type of equipment.
2. Description of the Prior Art
Time division hybrid multiplex data arrangements are already known that are intended for either circuit-mode or packet-mode switching (see Design Approaches and Performance Criteria for Integrated Voice/Data Switching by Myron J. Ross, Arthur C. Tabbot and John A. Waite, Proceedings of the IEEE, Vol. 65, No. 9 of September 1977). The time-interval distribution of a hybrid frame containing sample words and packets is depicted in FIG. 1A. F represents the frame, FL the frame limit indicator and L the limit between the synchronous time slot part of the frame containing time slots TS.sub.i and the channel .DELTA. intended for the packets.
In that part of the frame given over to the time slots (from FL to L), each time slot is allocated to a communication and only one. The bit-rate in a slot is thus guaranteed and characterized by the slot length. The various time slots TS.sub.1, TS.sub.i, TS.sub.I constitute a synchronous time-division multiplex.
That part of the frame reserved for the packets (from L to FL) is divided between several packet-mode communications channels. The packet making up one and the same communication are indicated by a packet number that they carry together with the data. This is the case of an asynchronous time division multiplex.
Described in U.S. patent application No. 210,819 filed Nov. 25, 1980 is a multiprocessor system comprising a plurality B of buses, a plurality of at the most B(B-1)/1 )/2 microprocessors each connected to a bus pair where each of the pairs connecting the multiprocessors are different and each bus is connected to at the most (B-1) microprocessors. It results from this that an originating microprocessor is connected directly via its two connection buses to 2(B-2) terminating microprocessors and indirectly to (B-2) (B-3)/2 terminating microprocessors via at the most one transit or relay microprocessor directly connected to both the originating microprocessor and the terminating microprocessor. Consequently, considering one originating microprocessor amongst the B(B-1)/2 microprocessors, there are:
B(B-1)/2-1=(B-2) (B+1)/2 terminating microprocessors possible. Out of these (B-2) (B+1)/2 terminating microprocessors, 2(B-2) are wired directly to the originating microprocessor and (B-2) (B-3)/2 are wired to it indirectly via a single relay microprocessor. It is confirmed that ##EQU1##
The system that has just been summarized for recap purposes affords numerous advantages, notably:
By taking as address of a given microprocessor the concatenation of the two addresses of the buses that are connected to it, i.e. by taking (ab) or (ba) as the address of the microprocessor connected to buses a and b, the microprocessor that recognizes its address transmitted by a bus connected thereto knows that it is the terminating microprocessor and, further, if it recognizes only a or only b in (ab) or (ba), then it knows itself to be a relay microprocessor and automatically interconnects its two buses one to the other. This property will be brought into play hereinafter by expressing the addresses of the switching stations to which the multiplex highways are connected in the (x, y) form.
Means now exist for connecting thirty or so microprocessors to a series bus having a 10 Mbit/s bit-rate. Consequently, B1 =30 and B=31. The network has 31 buses to which can be connected a maximum of B(B-1)/2 =435 microprocessors. The maximum theoretical traffic is thus virtually 300 Mbit/s which permits a practical bit-rate of 200 Mbit/s.